Optical disk data storage systems allow the storage of large quantifies of data on a disk. The data is accessed by focusing a laser beam onto the data layer of the disk and then detecting the reflected light beam.
In write-once read-many (WORM) systems, the laser writes data by making permanent marks on the data layer. Once the data is recorded onto the disk it cannot be erased. The data in a WORM system is detected as a change in reflectivity between written and non-written regions of the data layer.
In an ablative WORM system, the laser writes data by melting portions of the data layer, resulting in physical pits in the data layer. In a phase-change WORM system phase-change alloys are used as the recording material and the laser writes data by locally converting the phase-change data layer from one structural phase (e.g., amorphous) to a second structural phase (e.g., crystalline). This is done when the amorphous region is heated and maintained at or above its crystallization temperature, or alternatively melted and slowly cooled until the region is crystallized. The WORM function is achieved because the medium is designed so that the second structural phase does not convert back easily to the first structural phase.
In contrast to ablative and phase-change systems, another type of WORM system uses an organic dye-based data layer. The dyes are defined generally as organic compounds that have conjugated double bonds, and include materials from the dye families of cyanine, squarylium, phthalocyanine, azomethine, merocyanine, anthraquinone and phenylenediamine metal complexes. The marks are written in the dye-based data layer by heating a small region above the decomposition temperature of the dye to cause a local change in the optical properties of the material. The primary effect of the laser heating is a change in the index of refraction of the dye material, although in certain dye materials there may also be some ablation or deformation of the material. The result is that there is a change in reflected light intensity between the written and non-written regions of the dye-based data layer.
To increase the storage capacity of an optical disk, multiple data layer systems have been proposed. An optical disk having two or more data layers may be accessed at the different spatially separated data layers by changing the focal position of the lens. Examples of this approach are described in U.S. Pat. Nos. 5,202,875; 5,097,464; and 4,450,553. One problem with using multiple data layers in dye-based WORM disks is that the conventional dye WORM materials are highly absorptive of light. Multiple data layer disks require that the intermediate data layers between the disk surface onto which the laser light is incident and the last or farthest data layer from that surface be light transmissive. Because the conventional dye-based WORM materials tend to absorb a high percentage of the light, it is not possible to write on the farther data layers. If the intermediate data layers are merely made thinner to make them transparent, they then lack sufficient reflectivity and/or signal contrast (the difference in reflectivity between the written and non-written regions) so that they fail to function as dye-based WORM data layers.
Multiple data layer dye-based optical disk systems have been proposed, as described, for example, in U.S. Pat. No. 4,090,031 and Japanese published patent application JP05-151616. However, in these systems each dye-based data layer is sensitive to light at a different wavelength. This requires that the different data layers be formed of different dye materials and that the optical drive include light filters or multiple laser sources to generate light at the different wavelengths.
IBM's co-pending application Ser. No. 08/167,714, filed Dec. 15, 1993, relates to a multiple data layer CD-ROM type optical disk wherein dye material is used in the reflective layers that are coated onto the embossed patterns of pits that form the CD-ROM data layers.
What is needed is a multiple data layer dye-based WORM optical disk system that provides a good signal from the data layers closer to the incident laser light, that allows writing with reasonable laser power on all data layers, and that is operable at a single laser wavelength so that the data layers can be formed of substantially the same dye material.